Draft: remap implementation in std library

src/doc/stdlib.md

@@ -166,6 +166,12 @@ the computations are performed component-by-component (separately for `x`,
     $ x*(1-\alpha) + y*(\alpha) $
 
 
+*`type`* **`unmix`** (*`type`* `x`, *`type`* `y`, *`type`* `alpha`) <br> *`type`* **`unmix`** (*`type`* `x`, *`type`* `y, float alpha`)
+  : The `unmix` function returns a inverse of `mix`, similar to inverse linear interpolation:
+    $ (x - (\alpha)) / (y - x) $
+    The x and y value can't be the same, as we can't divide by zero.
+
+
 *`type`* **`select`** (*`type`* `x`, *`type`* `y`, *`type`* `cond`) <br> *`type`* **`select`** (*`type`* `x`, *`type`* `y, float cond`) <br> *`type`* **`select`** (*`type`* `x`, *`type`* `y, int cond`)
   : The `select` function returns `x` if `cond` is zero, or `y`
     if `cond` is nonzero. This is roughly equivalent to `(cond ? y : x)`,

src/shaders/color2.h

@@ -183,6 +183,12 @@ color2 mix(color2 a, color2 b, float x )
                   mix(a.a, b.a, x));
 }
 
+color2 unmix(color2 a, color2 b, float x )
+{
+    return color2(unmix(a.r, b.r, x),
+                  unmix(a.a, b.a, x));
+}
+
 color2 smoothstep(color2 edge0, color2 edge1, color2 c)
 {
     return color2(smoothstep(edge0.r, edge1.r, c.r),

src/shaders/color4.h

@@ -183,6 +183,12 @@ color4 mix(color4 a, color4 b, float x )
                   mix(a.a, b.a, x));
 }
 
+color4 unmix(color4 a, color4 b, float x )
+{
+    return color4(unmix(a.rgb, b.rgb, x),
+                  unmix(a.a, b.a, x));
+}
+
 color4 smoothstep(color4 edge0, color4 edge1, color4 c)
 {
     return color4(smoothstep(edge0.rgb, edge1.rgb, c.rgb),

src/shaders/stdosl.h

@@ -164,6 +164,32 @@ float  mix (float  x, float  y, float  a) BUILTIN;
 closure color mix (closure color x, closure color y, float a) { return x*(1-a) + y*a; }
 closure color mix (closure color x, closure color y, color a) { return x*(1-a) + y*a; }
 
+#if 0 // Assuming x != y
+normal unmix (normal x, normal y, normal a) { return (a - x) / (y - x); }
+normal unmix (normal x, normal y, float  a) { return (a - x) / (y - x); }
+vector unmix (vector x, vector y, vector a) { return (a - x) / (y - x); }
+vector unmix (vector x, vector y, float  a) { return (a - x) / (y - x); }
+point  unmix (point  x, point  y, point  a) { return (a - x) / (y - x); }
+point  unmix (point  x, point  y, float  a) { return (a - x) / (y - x); }
+color  unmix (color  x, color  y, color  a) { return (a - x) / (y - x); }
+color  unmix (color  x, color  y, float  a) { return (a - x) / (y - x); }
+float  unmix (float  x, float  y, float  a) { return (a - x) / (y - x); }
+#else
+normal unmix (normal x, normal y, normal a) BUILTIN;
+normal unmix (normal x, normal y, float  a) BUILTIN;
+vector unmix (vector x, vector y, vector a) BUILTIN;
+vector unmix (vector x, vector y, float  a) BUILTIN;
+point  unmix (point  x, point  y, point  a) BUILTIN;
+point  unmix (point  x, point  y, float  a) BUILTIN;
+color  unmix (color  x, color  y, color  a) BUILTIN;
+color  unmix (color  x, color  y, float  a) BUILTIN;
+float  unmix (float  x, float  y, float  a) BUILTIN;
+#endif
+// closure color unmix (closure color x, closure color y, float a) { return (a - x) / (y - x); }
+// closure color unmix (closure color x, closure color y, color a) { return (a - x) / (y - x); }
+
+// TODO: ADD REMAP FROM MIX AND UNMIX
+
 normal select (normal x, normal y, normal cond) BUILTIN;
 vector select (vector x, vector y, vector cond) BUILTIN;
 point  select (point  x, point  y, point  cond) BUILTIN;

src/shaders/vector2.h

@@ -182,6 +182,11 @@ vector2 mix(vector2 a, vector2 b, float x )
     return vector2 (mix(a.x, b.x, x), mix(a.y, b.y, x));
 }
 
+vector2 unmix(vector2 a, vector2 b, float x )
+{
+    return vector2 (unmix(a.x, b.x, x), unmix(a.y, b.y, x));
+}
+
 float dot(vector2 a, vector2 b)
 {
     return (a.x * b.x + a.y * b.y);

src/shaders/vector4.h

@@ -209,6 +209,14 @@ vector4 mix(vector4 value1, vector4 value2, float x )
                     mix( value1.w, value2.w, x));
 }
 
+vector4 unmix(vector4 value1, vector4 value2, float x )
+{
+    return vector4 (unmix( value1.x, value2.x, x),
+                    unmix( value1.y, value2.y, x),
+                    unmix( value1.z, value2.z, x),
+                    unmix( value1.w, value2.w, x));
+}
+
 vector vec4ToVec3(vector4 v)
 {
     return vector(v.x, v.y, v.z) / v.w;